Tam Mamuang - replace papaya with raw mango (Mamuang)... often made in Som Tam Thai style
Tam Tang - replace papaya with cucumber
Tam Apple - usually green apple. This dish is quite popular among Thai students abroad who are desperate for Sam Tam (well I was one of them back then!)
Tam Carrot
The best Som Tam in Thailand that I've ever had is in Udonthani - the province in Esarn not far from Laos border. That was Som Tam Poo Pla Raa by default if you are around that area